The invention relates to a process and apparatus for treating a pulp web in a paper or board machine in a long-nip pressing unit with one press roll that has a rotating pressing shell and one opposing roll, where the pulp web is dewatered in an extended press nip between the opposing roll and the pressing shell of the press roll.
In conventional paper and tissue production processes, mechanical dewatering of a pulp web before thermal drying takes place by direct pressing of the pulp web onto a drying cylinder (Yankee cylinder). A production process of this kind is described in DE 102 33 920 A1. In these paper and tissue machines, however, mechanical pressing and the line load that can be achieved is limited by the pressure because it is applied to the Yankee drying cylinder. By means of a preceding pressing stage, as described in EP 1 075 567 B1 for example, mechanical dewatering is performed in a press unit that is independent of the Yankee cylinder. Here, it is possible to set optimum pressing conditions because the pressure is no longer applied to the drying cylinder and thus is not limited by the load limits of the drying cylinder. Mechanical dewatering can be improved substantially by this preceding pressing stage, which is preferably carried out in a ling-nip pressing unit, particularly a shoe press. The effort involve in thermal drying is reduced, thus leading to energy savings.
EP 1 397 553 B1 describes a process to produce a pulp web, where the pulp web is dewatered by means of a shoe press before being transferred to a through-air drum (TAD) for the thermal drying.
In conventional processes for producing board, it is state of the art to use one or several separate pressing stages for mechanical dewatering before thermal drying. A press arrangement of this kind is described, for example, in EP 0 954 634 B1. Here, too, the pressing steps are often carried out with one or more extended-nip pressing units, such as shoe presses.
In order to be dewatered in an extended-nip pressing unit, such as a shoe press, the pulp web is fed on a felt through a press nip, which is formed by a shoe press roll with a rotating pressing shell and an opposing roll. After the extended press nip, the felt is separated from the pulp web as rapidly as possible. The pulp web then continues on a fabric that was also fed through the extended press nip or is carried onward by the opposing roll.
The pressing shell of extended-nip pressing units has direct contact with a fabric in the extended press nip in conventional plants and is used solely for mechanical dewatering.
Onward transport of the pulp web is always performed by other structural elements.